1. Field of the Invention
The present invention relates to a recording apparatus that executes recoding on a sheet by discharging liquid.
2. Description of the Related Art
FIGS. 14 and 15 illustrate a conventional ink jet recording apparatus. A conveyance roller 1, a pinch roller 2, a platen 3, and a cutter 10 are sequentially arranged in the traveling direction of a roll sheet P1 conveyed through a roll sheet conveyance path from an automatic document feeder. The conveyance roller 1 and the pinch roller 2 convey the roll sheet P1 conveyed through the roll sheet conveyance path to an image recording unit to execute recording.
The image recording unit includes a platen 3 that suctions and supports the roll sheet P1 and a recording head 4 that is arranged opposite the platen 3, moves in a main-scanning direction, and discharges ink on the roll sheet P1 on the platen 3 to execute recording. The recording head 4 has a plurality of nozzle arrays (not illustrated) discharging ink on a surface facing the roll sheet P1. These nozzle arrays are arrayed in a sub-scanning direction, and each nozzle array discharges ink of a different color. The nozzle arrays are supplied with ink of the respective colors from respective ink tanks via respective supply tubes.
A carriage 16 including the recording head 4 is slidably supported by a guide shaft 18 and a guide rail 19 parallel to each other. Both ends of the guide shaft 18 and the guide rail 19 are fixed to frames of a printer body 17. A belt drive device and a motor (not illustrated) can reciprocally move the carriage 16 in the main-scanning direction along the guide shaft 18 and the guide rail 19.
When the ink jet recording apparatus records an image, the recording head 4 first executes recording corresponding to a single scanning line, and then temporarily stops the printing operation. After the conveyance roller 1 conveys the roll sheet P1 by a predetermined length, the recording head 4 again executes recording corresponding to the next single scanning line. Thus, by reciprocally moving the carriage 16 and discharging ink from the recording head 4, the ink jet recording apparatus records a desired image on the roll sheet P1. When a cut sheet is used, a discharge roller 30 conveys the trailing edge of the cut sheet.
Next, a configuration of the platen 3 will be described with reference to FIG. 16. Sheet suction portions 5 for suctioning and supporting the roll sheet P1 on the platen 3 include ribs 11 formed at intervals in the main-scanning direction of the carriage 16. On a bottom surface of a recessed portion 12 arranged between each pair of ribs 11, a suction hole 7 is formed. Each suction hole 7 is in communication with a suction force generation unit and keeps the recessed portions 12 in negative pressure to prevent floating of the roll sheet P1.
If the interval between each pair of ribs 11 is increased, the roll sheet P1 sinks between the ribs 11, and the recording head 4 is less likely to rub against the roll sheet P1. However, if the roll sheet P1 excessively sinks between the ribs 11, visible marks are left on the printed roll sheet P1, resulting in degradation of image quality. Thus, an optimum interval value between each pair of ribs 11 is a very important parameter to keep image quality.
In addition, ink receiving portions for borderless printing are arranged on the platen 3. Japanese Patent Application Laid-Open No. 2006-231612 discusses a configuration of ink receiving portions. These ink receiving portions need to have a certain width in the main-scanning direction, being reserved for dealing with misalignment of sheet edges caused when a sheet is set, for example.
According to Japanese Patent Application Laid-Open No. 2006-231612, a plurality of ink receiving portions are arranged, so that when printing is executed on a large-width sheet, the ink receiving portions for smaller-width sheets are covered by the large-width sheet.
As illustrated in FIGS. 17A and 17B, ribs are also arranged on both sides of each ink receiving portion in the main-scanning direction, and the interval of these ribs is greater than the optimum interval of the other ribs corresponding to the sheet suction portions 5 (the portions other than the ink receiving portions on the platen 3). Each of the ink receiving portions needs to have a sufficient width to receive discharged ink that is intentionally caused to run off the sheet during borderless printing.
Further, when a roll sheet is set, the roll sheet could be misaligned and conveyed with some misalignment amount in the sheet width direction. For such a case, each of the ink receiving portions needs to have an extra width. In addition, to prevent running-off ink from flowing into the sheet suction portions 5, each of the ink receiving portions needs to have a minimum extra width.
Thus, when a large-width sheet is printed, the sheet can sink between ribs on both sides of an ink receiving portion for a small-width sheet. In reaction to this sinking portion, the portions adjacent thereto are easily floated. Even if the sheet does not sink, due to significant changes in the rib pitch, the expansion amounts of the sheet swelled by ink can be collected. These factors cause various problems such as irregular wavy marks left visible near the ink receiving portions, image unevenness due to a decrease in accuracy of ink placement, or friction produced between the recording head and the sheet.